I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and system for providing personal base station communications within the coverage area of a cellular base station.
II. Description of the Related Art
As wireless communication systems become more prevalent in society, the demands for greater and more sophisticated service have grown. To meet the capacity needs of wireless communication systems, techniques of multiple access to a limited communication resource have been developed. The use of code-division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access techniques, such as time-division multiple access (TDMA) and frequency-division multiple access (FDMA) are known in the art. However, spread-spectrum modulation techniques of CDMA have significant advantages over these other modulation techniques for multiple access communication systems.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference. The use of CDMA techniques in a multiple access communication system is also further disclosed in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference.
The teachings of the just mentioned patents have been applied to relatively large wireless communication systems such as cellular telephone systems that, in turn, interface with a public switched telephone network (PSTN). In this way, the user of a subscriber station such as a cellular telephone may generally originate or receive calls from any other communication device connected to the PSTN as long as the subscriber station is located within the geographical coverage area of any wireless base station belonging to the cellular system. The coverage area for these base stations generally extends several miles. The base stations of these cellular systems are generally referred to as xe2x80x9cmacroxe2x80x9d base stations, and their respective cell sites as xe2x80x9cmacroxe2x80x9d cell-sites.
Due to the relatively high cost of cellular telephone service through these macro base stations as compared to traditional landline telephone service, it is not presently cost effective to use a cellular telephone for all of one""s desired telephone communications. Thus, users of cellular telephones generally use the cellular telephone only when a convenient landline connection is not available, such as when they are away from their home or office. This leads to inconvenience as the user must switch between phones when entering or leaving his home or office.
Some prior art wireless telephones have been suggested which operate in a dual-mode cellular/cordless manner in a common handset. These prior art wireless telephones provide cellular service to the PSTN through the macro cells of a cellular communication system, and cordless service to the PSTN through a xe2x80x9cmicroxe2x80x9d base station such as a standard cordless telephone base unit. The dual-mode cellular/cordless handset automatically switches between the standard cellular mode of operation and the cordless mode of operation as the user transitions into the coverage area of the micro base station. Thus, when the user is away from home, he uses the dual-mode phone in the cellular mode, and incurs cellular service charges. However, when the user is within the coverage area of the cordless telephone base unit, typically inside of the home or office, he uses the dual-mode phone in the cordless mode, avoiding the cellular service charges.
A problem with the prior art solution is that since the dual-mode telephones typically must operate on two different frequency bands and use two different communication protocols and modulation schemes, they must include additional costly components. For example, they typically include separate transmit and receive paths for the cellular and the cordless signals, complex switches and special control circuits. These additional components add cost, size and weight to the prior art dual-mode telephones.
What is needed is a communication system that simultaneously provides cellular service and local wireless service without increasing the cost or complexity of the subscriber station.
The present invention is a novel and improved method and system for providing personal base station communications within the xe2x80x9ccellxe2x80x9d of a cellular base station. As defined and used herein the term xe2x80x9ccellxe2x80x9d will refer to a geographical coverage area whereas the term xe2x80x9ccell-sitexe2x80x9d will be used to refer to the physical equipment used to perform communications, i.e. one or more base stations. The present invention provides a method and system for operation of a personal base station where the forward link (base station to subscriber station) of a personal base station is on the same frequency assignment as the forward link of a macro base station belonging to a cellular communication system. By operating the personal base station on the same frequency assignment as the macro base station, an operator is not required to use additional spectrum in order to support the micro base station. Since an operator has a fixed amount of spectrum allocated to it, and if the operator was using au of its existing spectra, the operator would have to undergo great expense to add more cells to free up a frequency. Other alternatives such as obtaining more spectrum are generally not available to an operator. Although the present invention is disclosed herein with reference to a CDMA system, it is understood that the teachings are equally applicable to other wireless communication schemes, whether digital or analog, and regardless of the modulation scheme employed.
In the present invention, a first wireless base station is operated on the same frequency band as a second wireless base station. The first wireless base station, a xe2x80x9cmacroxe2x80x9d base station, generates and transmits a first forward link data signal and communicates with a first subscriber station. A second wireless base station, a xe2x80x9cmicroxe2x80x9d base station, generates a second forward link data signal and communicates with a second subscriber station. The second wireless base station receives the first forward link data signal and combines it with its own second forward link data signal to form a combined forward link data signal. The second wireless base station then transmits the combined forward link data signal. Thus, the first subscriber station, which is in communication with the macro base station, is able to receive and diversity combine the macro base station forward link data from the combined forward link data signal transmitted by the micro base station, improving the signal to noise ratio that would otherwise occur in the vicinity of the micro base station.
In a first embodiment of the present invention, the micro base station combines the first forward link signal with its own outgoing second forward link signal at radio frequency (RF). In a second embodiment of the present invention, the micro base station combines the first forward link signal with its own outgoing second forward link signal at an intermediate frequency (IF).
The present invention also delays the received first forward link data signal for a predetermined delay period prior to combining it with the second forward link data signal, so that it appears to the first subscriber station as a resolvable multipath signal. In order to avoid self-interference, the second wireless base station switches between receiving the first forward link data signal and transmitting the combined forward link data signal at a predetermined switching period. In the preferred embodiment, the predetermined switching period results in approximately a 50% transmit duty cycle. Thus, the micro base station does not transmit substantially continuously, but rather switches roughly on the xe2x80x9chalf-intervalxe2x80x9d of a predetermined time interval between transmitting a combined signal, and receiving the first forward link signal from the macro base station.
In another aspect of the present invention, a power measurer in the micro base station measures a power level of the delayed received first forward link data signal and a gain adjuster adjusts the power level of the delayed received first forward link data signal in response to the power level measurement in order to scale the first forward link data signal with respect to the second forward link data signal. In the preferred embodiment, the scaling factor is determined in accordance with the received power of the first forward link signal as measured by the power measurer. This scaling is performed in order to ensure sufficient energy of the retransmitted macro base station forward link data at the first subscriber station without unduly degrading the signal to noise ratio of the micro base station""s own forward link data at the second subscriber station.
In accordance with another aspect of the present invention, unacceptable interference from the second subscriber station, which is communicating with the micro base station, is avoided by the micro base station either terminating communication with second subscriber station or executing a handoff of the second subscriber station to the macro base station when the transmit power of the second subscriber station exceeds a predetermined threshold. In this regard, a power control command generator in the micro base station generates power control commands, each of the power control commands indicating an increase or decrease in transmit power. A transmitter in the micro base station transmits these power control commands to the second subscriber station. To avoid excessive interference, the micro base station terminates communication with the second subscriber station if the micro base station transmits a predetermined number of consecutive power control commands indicating an increase in transmit power. In an alternative embodiment, the base station informs the second subscriber station of the maximum power that the second subscriber station using the micro base station is allowed to transmit. The second subscriber station is not allowed to exceed this power while communicating with the micro base station. When the second subscriber station using the micro base station reaches this limit, the micro base station will continually send power control commands to have the second subscriber station increase its output power; however, the second subscriber station does not increase its transmit power. The micro base station can then sense that the second subscriber station is at the edge of coverage and release the call. The micro base station can set the maximum amount of power that the second subscriber station is allowed to transmit by monitoring the amount of power which is received from the macro base station.
In accordance with another aspect of the present invention the macro base station typically includes means for maintaining extremely accurate time and frequency reference. This is generally accomplished by means of a Global Positioning System (GPS) satellite receiver or other expensive equipment. However, it may be prohibitively expensive to provide such precision equipment at the micro base station. Thus, in the present invention, the micro base station obtains the accurate time and frequency reference from the macro base station. In this regard, the micro base station includes a demodulator which demodulates the received first forward link data signal, and time reference determining means for determining a time reference from the demodulated received first forward link data signal. Furthermore, the micro base station includes frequency reference determining means for determining a frequency reference from the demodulated received first forward link data signal.